Intensity controlling circuit having fuses and EPROMs for LED-array head

ABSTRACT

An intensity controlling circuit device can correct variation in intensity of light beams, due to tolerance occurred in each of a plurality of LED-array chips, emitted by LEDs provided in each of the LED-array chips. The intensity controlling circuit device is connected to at least one LED-array chip comprising a plurality of LEDs and slave transistors corresponding to each of the LEDs. The intensity controlling circuit device comprises an intensity controlling circuit connected to the respective LED-array chip. The intensity controlling circuit comprises a first transistor provided between a power source and a constant current generating unit so as to supply a current to the LED-array chip, and an intensity adjusting unit having a second transistor connected to the first transistor in parallel and a controlling unit for controlling the on/off state of the second transistor. An output of the intensity controlling circuit is connected to each of the slave transistors provided in the LED-array chip, and the second transistor is turned on/off by the controlling unit so that a current flowing in each of the LEDs is adjusted to a predetermined value.

BACKGROUND OF THE INVENTION

The present invention relates to an intensity controlling circuit devicefor an LED-array head used for printers or facsimile machines.

A light emitting diode (LED) is used for projecting a light beam so asto expose a photosensitive material of a printer drum provided in aprinter. Intensity of the LED may be controlled using an intensitycontrolling circuit device 6 shown in FIG. 1. In the device 6 shown inFIG. 1, a few thousands to a few ten thousands of LEDs 1-1 to 1-n may beused. P-channel type MOS (PMOS) transistors 2-1 to 2-n, which are slavetransistors, are connected to anodes of the respective LEDs 1-1 to 1-n.A gate of each transistor 2-1 to 2-n is connected to a power source 8via respective PMOS transistors 7-1 to 7-n. The gate of each transistor2-1 to 2-n is also connected to an output of the intensity controllingcircuit device 6 via respective transmission gates 5-1 to 5-n. Outputsof AND circuits 9-1 to 9-n are connected to gates of the PMOStransistors 7-1 to 7-n and control terminals of the transmission gates5-1 to 5-n. The AND circuits 9-1 to 9-n are connected to outputs oflatch 4. The latch 4 latches binary image data, which is stored in ashift register 3, in synchronization with a shift clock signal suppliedto the shift register 3. Each of the AND circuits 9-1 to 9-n suppliesthe binary image data to the respective LEDs 1-1 to 1-n according to anenable signal supplied to an input thereof.

In the above-mentioned circuit, data according to the binary image datalatched by the latch 4 is output from the AND circuits 9-1 to 9-n whilethe enable signal supplied to the AND circuits 9-1 to 9-n is in a highlevel state. The slave transistors 2-1 to 2-n are selectively operatedto supply an electric current I2 to the respective LEDs 1-1 to 1-n sothat the selected LEDs 1-1 to 1-n emit light beams.

A construction of the intensity controlling circuit 6 is also shown inFIG. 1. The intensity controlling circuit device comprises a PMOStransistor 11, an N-channel type MOS (NMOS) transistor 12, a resistor 13and an operational amplifier 14. A source of the PMOS transistor 11 isconnected to the power source 8. A gate of the PMOS transistor 11 isconnected to the PMOS transistors 2-1 to 2-n. A gate of the NMOStransistor 12 is connected to an output of the operational amplifier 14.A reference voltage (Vref) is applied at a positive terminal of theoperational amplifier 14. A voltage applied to a point A in the figureis supplied to a negative terminal of the operational amplifier 14. Aconnecting point, indicated by B in the figure, between the PMOStransistor 11 and the NMOS transistor 12 is connected to the inputs ofthe transmission gates 5-1 to 5-n. The resistor 13 is connected betweenthe NMOS transistor 12 and the ground. The NMOS transistor 12, theresistor 13 and the operational amplifier 14 together comprise aconstant current generating means.

The operational amplifier 14 controls a voltage applied to the gate ofthe NMOS transistor 12 so that the voltage at the point A is equal tothe reference voltage Vref. Since the point A is grounded via theresistor 13, a current I1 (=Vref÷resistance of the resistor 13) flows tothe resistor 13. The current I1 flows into a drain of the NMOStransistor 12 via a drain of the PMOS transistor 11.

Since the PMOS transistor 11 and each of the PMOS transistors 2-1 to 2-ncomprises a current mirror circuit, a gate voltage of the PMOStransistor 11 is applied to a gate of each of the PMOS transistors 2-1to 2-n via the respective transmission gates 5-1 to 5-n. Accordingly, ifeach of the PMOS transistors 2-1 to 2-n has the same transistor sizewith the PMOS transistor 11, a current equal to the current I1 flowingin the PMOS transistor 11 flows to the PMOS transistors 2-1 to 2-n.Accordingly, a current flowing to each of the LEDs 1-1 to 1-n can becontrolled by the current flowing in the PMOS transistor 11. It shouldbe noted that the PMOS transistor 11 is referred to as a mastertransistor.

Additionally, Japanese Laid-Open Patent Application No. 63-240168discloses, with reference to FIG. 3, a circuit for adjusting anintensity of a light beam emitted by each LED provided in an LED-array.In the LED-array, a plurality of LEDs are arranged along a line. Thiscircuit is provided for adjusting the intensity of light beam emitted byeach of the LEDs so as to correct variation in intensity of the lightbeam due to tolerance of each LED.

In the circuit shown in FIG. 3 of Japanese Laid-Open Patent ApplicationNo. 63-240168, an LED driving transistor and an AND circuit are neededfor each of the LEDs. This construction causes an increase in size ofthe LED-array chip. Additionally, reliability of the LED-array chip isreduced due to an increase in the number of gates provided in theLED-array chip.

For example, a conventional LED-array head has 64 LEDs arranged in a 5mm square chip (LED-array chip). In an LED-array head used for A1 sizeprinter, approximately 300 LED-array chips are arranged along a line.The above-mentioned intensity controlling circuit 6 formed in the LEDdriving circuit chip is also provided to each of the LED-array chips. Inthe above-mentioned circuit construction of the LED-array head, all thetransistors and AND circuits comprising together the LED driving circuitmust be operated in a normal condition. Accordingly, an inspection timefor the LED-array head may be increased, and possibility of defect ofthe transistors and AND circuits may be increased, and thus reliabilityof the LED-array chip is decreased.

However, since LEDs and LED driving circuits have recently beenmanufactured using a wafer manufacturing process, the tolerance in theLEDs in the LED-array and the LED driving circuits can be almost zero.Therefore, recently, there has been no need to adjust the intensity ofthe light beam emitted by each of the LEDs in the LED-array.

On the other hand, while the variation in the light beam emitted by theLEDs in a single LED-array is reduced, variation of the intensity of thelight beams due to tolerance in the LED-array chips and the LED drivingcircuit chips still exists in the order of a few percent to a few tenspercent. This variation in intensity of the light beams causes adeterioration in printing quality.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide an improvedand useful intensity controlling circuit device for a light emittingdevice used in a printer in which intensity controlling circuit devicethe above-mentioned problems are eliminated.

A more specific object of the present invention is to provide anintensity controlling circuit device which can correct variation inintensity of light beams, due to tolerance incurred in each of aplurality of LED-array chips, emitted by LEDs provided in each of theLED-array chips.

In order to achieve the above-mentioned objects, there is providedaccording to the present invention, an intensity controlling circuitdevice used for an LED-array head having at least one LED-array chipcomprising a plurality of LEDs and slave transistors corresponding toeach of the LEDs, the intensity controlling circuit device comprising anintensity controlling circuit connected to the respective LED-array chipso as to control intensity of light beams emitted by the LEDs, theintensity controlling circuit comprising:

a first transistor provided between a power source and constant currentgenerating means for generating a constant current flowing in the firsttransistor so as to supply a current to the LED-array chip; and

intensity adjusting means having a second transistor connected to thefirst transistor in parallel and controlling means for controlling theon/off state of the second transistor,

wherein an output of the intensity controlling circuit is connected toeach of the slave transistors provided in the LED-array chip, and thesecond transistor is turned on/off by the controlling means so that acurrent flowing in each of the LEDs is adjusted to a predeterminedvalue.

According to the present invention, since the first and secondtransistors and the slave transistors constitute a current mirrorcircuit, the current flowing in the LEDs in the LED-array chip can becontrolled by each LED-array chip by turning on/off said secondtransistor. Therefore, variation in intensity of the light beams emittedby the LEDs in the LED-array chip can be corrected for each LED-arraychip.

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a conventional LED-array head includingan intensity controlling circuit device;

FIG. 2 is a circuit diagram of an embodiment of an intensity controllingcircuit device according to the present invention;

FIG. 3 is a circuit diagram of an LED driving circuit connected to theintensity controlling circuit shown in FIG. 2;

FIG. 4 is a block diagram of an LED-array head provided with theintensity controlling circuit device shown in FIG. 2;

FIG. 5 is an illustration for explaining a transistor size;

FIG. 6 is a circuit diagram of a variation of the embodiment of anintensity controlling circuit device shown in FIG. 2; and

FIG. 7 is a circuit diagram of another variation of the embodiment of anintensity controlling circuit device shown in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given, with reference to FIGS. 2 through 4, ofan embodiment according to the present invention. In FIGS. 2 throughFIG. 4, parts that are the same as the parts shown in FIG. 1 are giventhe same reference numerals, and descriptions thereof will be omitted.

Referring now to FIG. 4, which illustrates a circuit structure of anLED-array head having an embodiment of an intensity controlling circuitdevice according to the present invention, the intensity controllingcircuits 20-1 to 20-n, are connected to the respective LED drivingcircuits 40-1 to 40-n included in light emitting means. Hereinafter, theintensity controlling circuits 20-1 to 20-n may be referred to asintensity controlling circuits 20 as a whole, and the LED drivingcircuits 40-1 to 40-n may be referred to as LED driving circuits 40 as awhole. Each of the LED driving circuits 40 is connected to therespective LED-array chips 41-1 to 41-n, each of which is comprised of asingle chip. A plurality of LEDs are arranged in the LED-array chips41-1 to 41-n. The LED-array chips 41-1 to 41-n may be referred to asLED-array chips 41 as a whole. The above-mentioned light emitting meanscomprises, as shown in FIG. 4, the LED driving circuits 40 and thecorresponding LED-array chips 41.

Similarly to the conventional LED-array head shown in FIG. 1, ANDcircuits 9-1 to 9-n (may be referred to as AND circuits 9 as a whole)are connected to the respective LED driving circuits 40. Outputs oflatches 4-1 to 4-n (may be referred to as latches 4 as a whole) areconnected to inputs of the AND circuits 9. Outputs of the shiftregisters 3-1 to 3-n (may be referred to as shift registers 3 as awhole), which store image data supplied from an external device, areconnected to the respective latches 4.

In the present embodiment, latches 42-1 to 42-n (may be referred to aslatches 42 as a whole) are connected to outputs of the shift registers3. The latches 42 latch the data stored in the shift registers 3 when alatch enable signal is supplied from an external device. Each output ofthe latches 42 is connected to the corresponding control signal inputterminals 25-1 to 25-n provided in the intensity controlling circuits 20shown in FIG. 2.

When the latches 42 latch data, data stored in the shift registers 3 isnot the image data but the control signal data used for controllingintensity adjusting circuits 21 shown in FIG. 2. Accordingly, thelatches 42 latch the control signal data.

A first embodiment of an intensity controlling circuit device accordingto the present invention shown in FIG. 2 comprises the intensitycontrolling circuits 20, the LED driving circuits 40, the LED-arraychips 41, the latches 42 and the shift registers 3. It should be notedthat, for example, the intensity controlling circuit 40-1, the LED-arraychip 41-1, the latch 42-1 and the shift register 3-1 may be incorporatedinto one chip as an LED driving chip.

In the intensity controlling circuit 20 shown in FIG. 2, voltageindicated by "VG" corresponds to the voltage "VG" shown in FIG. 1.Additionally, the present embodiment has an operation amplifier 14 andthe resistor 13 similarly to the conventional intensity controllingcircuit 6 shown in FIG. 1.

As shown in FIG. 2, a connection point B between a PMOS transistor 11',which is a first transistor, and the NMOS transistor 12 is connected toan input of the LED driving circuit 40. Drains of PMOS transistors 23-1to 23-4 and inputs of transmission gates 24-1 to 24-4 are also connectedto a signal line extending from the connection point B to an inputterminal of the LED driving circuit 40. The PMOS transistors 23-1 to23-4, which are second transistors, act as intensity controllingtransistors. The transmission gates 24-1 to 24-4 are turned on/offaccording to control signals supplied via control signal input terminals25-1 to 25-4 so as to turn on/off the PMOS transistors 23-1 to 23-4.

Outputs of the transmission gates 24-1 to 24-4 are connected to drainsof PMOS transistors 26-1 to 26-4 and drains and gates of the PMOStransistors 23-1 to 23-4. Sources of the PMOS transistors are connectedto the power source 8. Gates of the PMOS transistors 26-1 to 26-4 areconnected to the control signal input terminals 25-1 to 25-4. Asmentioned above, in the present embodiment, the latches 42 are connectedto the control signal input terminals 25-1 to 25-4 so that the intensitycontrolling circuits 20 are connected to the latches 42.

It should be noted that intensity controlling means for controllingon/off of the transistors 23-1 to 23-4 comprises the transmission gates24-1 to 24-4, the control signal input terminals 25-1 to 25-4, the PMOStransistors 26-1 to 26-4 and inverters shown in FIG. 2. Additionally, asshown in FIG. 2, the intensity adjusting circuit 21 comprises theintensity controlling means, the power source 8 and the PMOS transistors23-1 to 23-4.

The LED driving circuit 40 has a circuit construction similar to thecircuit, shown in FIG. 1, including the LEDs 1, PMOS transistors 2, thetransmission gates 5, PMOS transistors 7 and the power source 8. 64 LEDsare, for example, provided in each of the LED driving circuits 40.

In the above-mentioned embodiment of the intensity controlling circuitaccording to the present invention, PMOS transistor 11' and PMOStransistors 23-1 to 23-4 included in the intensity adjusting circuit 21and the PMOS transistors 2 included in the LED driving circuits 40comprise a current mirror circuit. Accordingly, in the case where thosePMOS transistors have the same transistor size (described later), andwhen the PMOS transistors 23-1 to 23-4 are in the off state, a currentflowing in the PMOS transistor 11' is equal to a current flowing in thePMOS transistors 2 included in each of the LED driving circuits 40.

It should be noted that, in the present invention, the transistor sizesof the PMOS transistor 11' and the PMOS transistors 23-1 to 23-4 are inthe ratio of 5:1:1:1:1. Additionally, the transistor sizes of each ofthe PMOS transistors 2 and the PMOS transistor 11' are in the ratio of5:5 (1:1). The transistor size refers to, as shown in FIG. 5 which is aplane view of a MOS transistor formed on an IC substrate, a width W ofdiffusion areas provided for forming a source and a drain of atransistor when a width L of a polycrystalline silicon on which gates ofthe MOS transistor are formed is constant. That is, as shown in FIG. 5,the width W is a sum of a width a and a with b of the diffusion areas.

Variation of the intensity of a light beam emitted by each of theLED-array chips 41 is measured after the LED-array head is manufactured.The current flowing to each of the LEDs 1 is determined for each of theLED-array chips 41 so that variation in intensity of the light beamemitted by each of the LED-array chips 41 is eliminated. The currentvalue of the determined current may be stored in an EPROM provided, forexample, in a printer into which the intensity controlling circuitdevice is incorporated. The stored current value may be supplied to theintensity controlling circuit device from the EPROM when an operation ofthe printer is started. Binary data obtained based on the current valueis then stored in the respective shift resistors 3, and then supplied tothe latches 42.

The binary data, which is a control signal, supplied to the latches 42is then supplied to each of the control signal input terminals 25provided in the intensity adjusting circuits 21 comprising the intensitycontrolling circuits 20. The PMOS transistors 26-1 to 26-4 and thetransmission gates 24-1 to 24-4 are turned on/off according to the levelof the binary data supplied to the control signal input terminals 25-1to 25-4, and thereby the PMOS transistors 23-1 to 23-4 are turnedon/off.

Considering the transistor size mentioned above, when all of the PMOStransistors 23-1 to 23-4 are turned off according to the binary signalssupplied to the control signal input terminals 25-1 to 25-4, the currentI2 flowing in each of the LEDs 1 is equal to the current I1 because thetransistor size of the transistor 11' and each of the PMOS transistors 2are equal to each other as mentioned above (I2=(5/5)×I1).

For example, when one of the PMOS transistors 23-1 to 23-4 is turned onand others are turned off, the current I2 flowing in the LEDs 1 is equalto (5/6)×I1 because the ratio of the transistor sizes of the PMOStransistor 11' to each of the PMOS transistors 23-1 to 23-4 is 5:1. Thecurrent I2 is calculated by the equation I2={5/(5+1)}×I1.

For example, when two of the PMOS transistors 23-1 to 23-4 are turned onand others are turned off, the current I2 flowing in the LEDs 1 is equalto (5/7)×I1 because the ratio of the transistor sizes of the PMOStransistor 11' to each of the PMOS transistors 23-1 to 23-4 is 5:1. Thecurrent I2 is calculated by the equation I2={5/(5+2)}×I1.

Similarly, when three of the PMOS transistors 23-1 to 23-4 are turned onand one is turned off, the current I2 flowing in the LEDs 1 is equal to(5/8)×I1. When all of the PMOS transistors 23-1 to 23-4 are turned on,the current I2 flowing in the LEDs 1 is equal to (5/9)×I1.

As mentioned above, by controlling the on/off state of the transistors23-1 to 23-4 by the control signal supplied to the control signal inputterminals 25 via the latches 42, all of the LEDs 1 provided in each ofthe LED driving circuits 40 can be controlled. That is, intensity of thelight beams emitted by each of the LED-array chips 41 corresponding tothe respective LED driving circuits 40 can be adjusted in accordancewith the control signal supplied to the control signal input terminals25-1 to 25-4.

The above-mentioned adjustment of the light beams emitted by each of theLED-array chips 41 can be realized by merely adding the intensityadjusting circuits 21 each of which has a simple circuit construction.Accordingly, size of the LED driving chips is not greatly increased, andthus the manufacturing cost of the LED-array head can be reduced.Additionally, inspection of the LED driving circuits 40 can be completedby only performing an inspection for one of the PMOS transistors 2-1 to2-m because the same current always flows to each of the PMOStransistors 2-1 to 2-m in the same LED driving circuit. Accordingly,inspection time and cost for the LED-array head can be reduced. Further,since the number of gates connected to one LED in the LED-array chip isreduced as compared with the conventional apparatus, high reliability ofthe LED driving chip can be obtained.

In the above mentioned embodiment, the transistor sizes of the PMOStransistors 11' and 23-1 to 23-4 may instead be in the ratio of20:1:2:4:8. In this case, the ratio of the transistor size of the PMOStransistor 11' to each of the PMOS transistors 2 is set to 20:20 (1:1).By using the above-mentioned transistor sizes, the current I2 flowing inthe LEDs 1 can be set to sixteen levels, that are (20/20)×I1,(20/21)×I1, (20/22)×I1, (20/23)×I1, (20/24)×I1, . . . , (20/35)×I1.

Additionally, by changing the ratio of the transistor size of the PMOStransistor 11' to each of the PMOS transistors 2, the adjustment rangeof the current I2 can be wider or narrower. For example, when the ratioof the transistor size of the PMOS transistor 11' to each of the PMOStransistors 2 is set to 25:40, the above-mentioned current I2 can bechanged from (40/40)×I1 to (40/25)×I1.

It should be noted that although four PMOS transistors 23-1 to 23-4 areprovided in the above-mentioned embodiment, the number of PMOStransistors is not limited to the specific number and a less or greaternumber of PMOS transistors may be provided to decrease or increase thenumber of adjusting levels of the intensity of the light beams emittedby the LED-array chip. In such a case, the number of bits of the data tobe supplied to the latches 42 must be changed to an appropriate value.

In the above-mentioned embodiment, although the control signal issupplied to the control signal input terminals 25 via the latches 42,the control signal may be supplied directly only to the control signalinput terminals which are connected to the PMOS transistors 23-1 to 23-4to be controlled.

FIG. 6 shows a variation of the above-mentioned embodiment. In thisvariation, a fuse is provided instead of each of the control signalinput terminals 25. Each fuse may be blown off when the correspondingPMOS transistors 23-1 to 23-4 are turned on so as to increase theintensity of the light beams emitted by the LED-array chips 41.

FIG. 7 shows another variation of the above-mentioned embodiment. Inthis variation, an EPROM is provided instead of each of the controlsignal input terminals 25. The EPROM stores the control signal accordingto the predetermined level of the intensity of the light beams emittedby the LED-array chips 41, and supplies the stored control signal so asto control the on/off state of the corresponding PMOS transistors 23-1to 23-4 similarly to the above-mentioned embodiment.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

What is claimed is:
 1. In an intensity controlling circuit device usedfor an LED-array head having at least one light-emitting means having aplurality of LEDs and corresponding slave transistors, an intensitycontrolling circuit connected to the respective light-emitting means soas to control intensity of light beams emitted by the LEDs, saidintensity controlling circuit comprising:a) a first transistor; b) aconstant current generating means for generating a constant currentflowing in said first transistor so as to generate a voltage whichcontrols a current output from said slave transistors; and c) intensityadjusting means having:1) an output connected to each of said slavetransistors in the light-emitting means; 2) a second transistorconnected in parallel to said first transistor, between a power sourceand said constant current generating means; and 3) controlling means forcontrolling the on/off state of said second transistor by turning saidsecond transistor on/off so that a current flowing in each of said LEDsis adjusted to a predetermined value; wherein a fuse is connected tosaid controlling means so that a control signal used for turning on/offsaid second transistor is supplied to said controlling means via saidfuse, said fuse being blown when said second transistor is to bemaintained in the on state so as to decrease a current flowing in eachof the LEDs in a single light-emitting means.
 2. In an intensitycontrolling circuit device used for an LED-array head having at leastone light-emitting means having a plurality of LEDs and correspondingslave transistors, an intensity controlling circuit connected to therespective light-emitting means so as to control intensity of lightbeams emitted by the LEDs, said intensity controlling circuitcomprising:a) a first transistor; b) a constant current generating meansfor generating a constant current flowing in said first transistor so asto generate a voltage which controls a current output from said slavetransistors; and c) intensity adjusting means having:1) an outputconnected to each of said slave transistors in the light-emitting means;2) a second transistor connected in parallel to said first transistor,between a power source and said constant current generating means; and3) controlling means for controlling the on/off state of said secondtransistor by turning said second transistor on/off so that a currentflowing in each of said LEDs is adjusted to a predetermined value;wherein an EPROM is connected to said controlling means so that acontrol signal, stored in said EPROM, used for turning on/off saidsecond transistor, is supplied to said controlling means from saidEPROM.